1. Field of the Invention
The present invention relates to a highly stiff and highly damping cast iron superior in Young's modulus and vibration damping characteristics. When the cast iron of the present invention is used as, for example, structural materials of machine tools or high precision machine tools which need stiffness, or for structural materials of accurate-measuring instruments affected by problems concerning Young's modulus and vibration, the processing efficiency of these materials, accuracy of processed goods and degree of precision can be improved.
2. Description of the Related Art
Flake graphite cast iron which is relatively superior in vibration damping capability has been primarily used as structural materials of machine tools. Flake graphite cast iron has a combined type vibration-proof mechanism because it contains a large amount of flake graphite and therefore has higher damping capability than steel and the like. It also has the characteristics that it is advantageous in moldability and cost in the production of a large-scale structural material. In the meantime, studies have been made as to materials such as concrete type materials, natural granite and CFRP, each having high damping capability, in consideration of application of these materials to machine tool structural materials instead of using flake graphite cast iron. However, none of these materials has been put to practical use because each of these materials has problems concerning low stiffness, processability and costs.
Flake graphite cast iron superior in damping capability, casting capability and cost are widely used for structural materials such as beds, tables and columns of machine tolls. However, machine tools for the processing of materials hard to be processed need high stiffness enough to maintain deep cutting stably and high vibration damping capability to restrain the generation of harmful vibrations. In the case where further strict vibration damping capability is required in this manner, there is the case where the use of current flake graphite cast iron fails to obtain high processing efficiency and accuracy of processed goods because of the influence of vibration.
Because flake graphite cast iron such as FC300 which is currently used for machine tools and the like contains a lot of flake graphite which develops a combined type damping mechanism, it is a structural material superior in vibration damping capability among conventional materials. In order to improve the vibration damping capability of this flake graphite cast iron, it is only required to increase the amount of flake graphite. However, there is the problem that the dynamic Young's modulus (hereinafter referred to simply as “Young's modulus”) decreases along with increase in the amount of flake graphite cast iron. The adjustment of the amount of flake graphite cast iron can be controlled by the amount of C and Si. As to the structural materials of machine tools, it is necessary to increase the wall thickness of the structural material to keep its stiffness if Young's modulus decreases. This causes not only a worsening of the problem of structural design but also an increase in cost, and is therefore undesirable.
As a method of improving the vibration damping capability, a method is proposed in which bainite or martensite is formed as the base organization of flake graphite cast iron (Casting Engineering 68 [1996], 876). However, in these methods, Young's modulus decreases as the vibration damping capability is improved, and it is therefore difficult to make the both compatible. Also, methods for improving the vibration damping capability are disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 63-140064 (Patent Document 1), 2001-200330 (Patent Document 2) and 2002-348634 (Patent Document 3). A method for improving the logarithmic decrement and the like are described in any of Patent Documents 1 to 3.
In these Patent Documents 1 to 3, the results of measurement of the vibration damping capability are shown. However, because nothing is found to show the descriptions of Young's modulus, its value is unclear. Specifically, it is suspected that Young's modulus is not essential but the strength is regarded as important because Patent Documents 1 and 2 relate to brake materials. In, particularly, Patent Document 1, there is the description that the object of the invention is to provide a brake material having high strength equal to that of gray cast iron and more excellent damping capability than gray cast iron. In Patent Document 3, there is the description that an aluminum-containing damping cast iron is invented to improve damping capability also with a view to improving the damping capability of machine tools and precision processing instruments. Although it is essentially necessary to keep the stiffness of the structural materials to keep the mechanical precision, this is not shown in these References.
It is found from these Patent Documents 1 to 3 that the vibration damping capability can be improved by adding aluminum. These methods, however, differ from each other when they are precisely examined. Specifically, Patent Document 1 describes that a material which is superior in vibration damping capability and has high strength is obtained by heat-treating cast iron to which aluminum is added. Patent Document 2 reveals that Al is added to cure and a hyper-eutectic composition is used to increase the amount of graphite and to form fine pores, thereby improving the vibration damping capability. However, Young's modulus decreases greatly in this method. Patent Document 3 shows an example in which aluminum is added to improve the vibration damping capability. However, there is no description concerning Young's modulus in Patent Document 3. Specifically, these methods described in Patent Documents 1 to 3 do not necessarily attain the compatibility between Young's modulus and vibration damping capability and it is therefore necessary to further improve the vibration damping capability.